The present invention generally relates to packaging and more particularly to an apparatus and method for making unitary packages which hold a plurality of components, each package containing a load wrapped in a web of stretched film.
Case packing or boxing is a common way of shipping multiple unit products. The multiple unit products are generally stacked in a corrugated box or are wrapped with kraft paper with the ends of the kraft paper being glued or taped. Another way of shipping such products is by putting a sleeve or covering of heat shrinkable film around the products and shrinking the sleeve to form a unitized package. The use of heat shrinkable film is described in U.S. Pat. Nos. 3,793,798; 3,626,645; 3,590,509 and 3,514,920. A discussion of this art is set forth in U.S. Pat. No. 3,867,806.
The present invention does not require a structural seal and therefore can use any type of stretchable material. The invention is designed to function with stretchable film webs such as nylon, polypropylene, P.V.C., polybutylene, polyethylene or any copolymer or blends of the aforementioned stretchable films. The present inventive apparatus utilizes a fastening mechanism which effectively fastens a wrapping of collapsed film to an adjacent wrap while severing the trailing edge of the film web from the load after the load has been spirally wrapped to form a package overwrap.
The use of spiral wrapping machinery is well known in the art. One such apparatus is shown by U.S. Pat. No. 3,863,425 in which film is guided from a roll and wrapped around a cylindrical load in a spiral configuration. A carriage drives the film roll adjacent the surface of the load to deposit a spiral overwrap around the load and returns in the opposite direction to deposit another spiral overwrap around the load.
It has previously been disclosed in U.S. Pat. No. 3,788,199 to spirally wind tapes in a manner that they overlap each other to provide suitable space therebetween when breathability is required. In this reference, a heavy duty bag is prepared by spirally winding stretched tapes of synthetic resin in opposite directions, so that they intersect each other to form a plurality of superimposed cylindrical bodies which are bonded together to form a cylindrical network. The spirally wound inner and outer tapes of the superimposed cylindrical body intersect each other at a suitable angle, depending upon the application intended, the preferred embodiment having substantially equal longitudinal transfer strength. In this preferred embodiment the tapes intersect each other at an angle of about 90.degree.. The angle defined by the tapes constituting the cylindrical network may be determined by varying the interrelationship between the travelling speed of the endless belts carrying the tape and the rotating speed of the bobbin holders, which rotate a plurality of tape bobbins to deposit the tape onto the moveable belt. The previously indicated patents rely on heat shrink material, adhesives, a heat seal or the tacky nature of the film to hold the outer layer of wrap in a fixed position.
In U.S. Pat. No. 3,003,297 a complex cutting and holding mechanism is used to place tape on a box and cut it off with the process being repeated for each box.
Additional references of interest which are pertinent to rotatable drives for wrapping packages are disclosed in U.S. Pat. Nos. 3,820,451; 3,331,312; 3,324,789; 3,309,839; 3,207,060; 2,743,562; 2,630,751; 2,330,629; 2,054,603; and 2,124,770.
Other applications in packaging are shown by U.S. Pat. Nos. 3,514,920 and 3,793,798 in which heat shrink film is wrapped around a pallet supporting a plurality of cartons. A similar full web apparatus using a tensioned cling film is shown by U.S. Pat. No. 3,986,611 while another apparatus using a tacky P.V.C. film is disclosed in U.S. Pat. No. 3,795,086.
The elasticity of the film or netting holds the products under more tension than either the shrink wrap or the kraft wrap, particularly with products which settle when packaged. The effectiveness of stretched plastic film in holding a load together is a function of the containment or stretch force being placed on the load and the ultimate strength of the total layered film wrap. These two functions are determined by the modulus or hardness of the film after stretch has occurred and the ultimate strength of the film after application. Containment force is currently achieved by maximizing elongation until just below a point where break of the film occurs. Virtually all stretch films on the market today including products of Mobil Chemical Company (Mobil X, Mobil C, Mobil H), Borden Resinite Division (PS-26). Consolidated Thermoplastics, Presto, PPD, and others consistently stretched less than 30% in field applications because of irregularities in film braking systems. These systems depend upon friction either directly on the film through a bar assembly such as that used by the Radient Engineering Company or indirectly as that shown in U.S. Pat. No. 3,867,806 and 4,077,179.
All of these prior art apparatuses suffer from a severe limitation which relates to cost per unit load for film unitization. Friction brake devices do not maintain a consistent force. These brakes are subject to variation due to their physical construction and their sensitivity to speed change caused by passage of corners of the load, and the resultant sudden speed up and slow speed down of film unwind. A typical 40".times.48" pallet load will incur a surface speed change of more than 40% with each quarter turn. Higher turntable speeds of 12-18 RPM produce additional resonating forces which change with a roll consumption and its resultant weight change. Additional limitations on maximum elongation are caused by film roll imperfections and gauge variations which accentuate the force variations described above to produce film ruptures. Even though all of the film previously described carry manufacturer's specified elongation rates above 300%, these rates cannot be approached because of limitations imposed by friction-type constant force devices.
One problem with shrink and non-cling stretch film packaging in addition to the fact that they do not allow a lead to breathe is that the primary strength and reliability of the package is determined by the consistent quality of the seal. These seals depend on a careful maintenance of the sealing jaw and are never as strong as the film itself. The time that it takes to make the seals is a limiting factor on the possible speeds of most shrink systems with the additional problem that some stretchable materials, as for example, stretch netting, or narrow film width cannot be effectively heat sealed.
In view of the previously stated characteristics of film the previously noted stretch machines including machines manufactured by Lantech Inc.; Infra-Pak; PS & D; Radient Engineering, I.P.M.: and Mima have limited capabilities.
When high elongation rates are attempted, the forces frequently either disrupt the stacking pattern or pull the load off of the turntable.
In addition non-vertical sides and corners on an irregular load place extreme forces on a small area of film during stretching, thereby causing a partial rupture at a point well below the force achievable on a flat side. This partial rupture causes a transfer of force to the remaining portion of the web. This force is frequently sufficient to produce a "zippering" of the entire film web.
The present invention provides an apparatus and process which prestretches film before wrapping the film around a load so that the film may be stretched from 40 to 300% before it is wrapped around the load holding the load under compressive forces.
Most plastic films when stretched above their yield point gain significantly in modulus and ultimate strength. A typical polyethylene will multiply three times the ultimate strength in pounds per square inch of cross-sectional area after being elongated approximately 300%. This significant increase in strength begins approximately when the yield point is exceeded in elongation. The yield point is achieved between 15% and 40% stretch for virtually all stretch films being used today. Limitations of current friction-type constant force devices prevent current stretch wrap applications from achieving the higher levels of containment force and ultimate strength available in most plastic films. Achieving the higher elongation levels with the invention allows fewer revolutions of film for equipment holding power. These higher levels of stretch not only allow fewer revolutions of film but also less film by weight for each revolution. Cost savings of more than 50% will frequently result from using the present invention.
If irregular loads, unstable loads, or crushable loads are incurred, the film leaving the prestretching mechanism can be set to a surface speed equal to or slightly faster than the average surface speed of the pallet. Dramatic reduction of force on the film will give most of the advantages of high elongation rates without its crushing force. This reduction of force is caused by the non-elastic strain recovery because the yield point is exceeded and the rapid stress relaxation incurred at the high elongation level.
Very stable loads or loads not subject to crushing can take advantage of the maximum strength of the film by slowing the film leaving the prestretching mechanism to a surface speed below the average speed of the pallet load, to produce additional stretch forces over and above those achieved during the initial elongation.